Classes and Objects

Object-oriented programming groups related data and behavior together into a class — a blueprint — and each individual object made from it is called an instance.

Syntax

class ClassName:
    def __init__(self, ...):
        ...

Example

class User:
    def __init__(self, name, role):
        self.name = name
        self.role = role

    def greet(self):
        return f"Hi, I'm {self.name}"

ada = User("Ada", "admin")
print(ada.greet())
print(ada.name)

Expected output

Hi, I'm Ada
Ada

__init__ is the constructor — it runs automatically when you create a new instance (User("Ada", "admin")). Every method takes self as its first parameter, which refers to the specific instance the method is being called on.

Common mistakes

• Forgetting self as a method's first parameter, or forgetting to write self.name (accessing just name instead) inside a method.

Practice

Write a Rectangle class with width and height, and a method area() that returns width * height.

class Rectangle:
    def __init__(self, width, height):
        self.width = width
        self.height = height

    def area(self):
        return self.width * self.height

r = Rectangle(4, 5)
print(r.area())